Systems are known in the state of the art for fixing surgical implants, which have a polyaxial screw coupled with an overlying element, known in the state of the art as a tulip. The tulip has a substantially cup-shaped conformation and has an upper free end that is open and has lateral through U-shaped slots and a lower end, hollow inside, having a through-hole. The lower end houses, within it, the head of the polyaxial screw so that the connecting part between the head of the polyaxial screw and the stem of the polyaxial screw occupies the through-hole so that the stem of the polyaxial screw extends outside the lower end of the tulip. The head of the polyaxial screw is retained within the lower end of the tulip, free to rotate. Within the tulip, above the head of the polyaxial screw, a connection bar is inserted radially into the tulip and coming out of it through the U-shaped slots, adapted to connect various systems for fixing surgical implants to one another during the placement of the implants. Above the connection bar, by means of a threaded coupling, a set screw is coupled to the tulip, adapted to press against the connection bar in order to bring it into contact with the head of the polyaxial screw and transmit the pressure imposed by the set screw to it, so that the head of the polyaxial screw is blocked in the desired position and is not, therefore, able to rotate further with respect to the tulip.
The system for fixing surgical implants described above is usually provided for its placement already assembled, i.e. with the head of the polyaxial screw coupled to the tulip. The surgeon has the task of inserting the connection bar and proceeding with the definitive assembly through the set screw.
For coupling the tulip to the head of the polyaxial screw, therefore, the known technique envisions inserting the stem of the screw through the upper free end of the tulip itself and making the stem of the polyaxial screw pass through the central body of the tulip until it comes out of the hole provided in the lower end. In this way, the stem of the polyaxial screw protrudes for the whole of its length from the lower end of the tulip, while the head of the polyaxial screw, due to its larger dimensions with respect to its stem, remains retained within the tulip. To prevent the polyaxial screw coming out of the top of the tulip a blocking element is used that acts directly on the head of the screw to prevent its axial movement but that is able to allow its rotation.